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When an excess of uronic acid over Cr~(VI) is used, the oxidation of D-glucaric acid (Glucar) by Cr~(VI) yields D-arabinaric acid, CO_2 and Cr~(III)-Glucar complex as final redox products. The redox reaction involves the formation...
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When an excess of uronic acid over Cr~(VI) is used, the oxidation of D-glucaric acid (Glucar) by Cr~(VI) yields D-arabinaric acid, CO_2 and Cr~(III)-Glucar complex as final redox products. The redox reaction involves the formation of intermediate Cr~(IV) and Cr~V species. The reaction rate increases with [H~+] and [substrate]. The experimental results indicated that Cr~(IV) and Cr~V are very reactive intermediates since their disappearance rates are much faster than Cr~(VI). Cr~(IV) and Cr~V intermediates are involved in fast steps and do not accumulate in the redox reaction of the mixture Cr~(VI)-Glucar. Kinetic studies show that the redox reaction between Glucar and Cr~(VI) proceeds through a mechanism combining one- and two-electron pathways: Cr~(VI) → Cr~(IV) → Cr~(II) and Cr~(VI) → Cr~(IV) → Cr~(III). After the redox reaction, results show a slow hydrolysis of the CrIII-Glucar complex into [Cr(OH_2)_6]~(3+). The proposed mechanism is supported by the observation of free radicals, CrO_2~(2+) (superoxo-Cr~(III) ion) and oxo-Cr~V-Glucar species as reaction intermediates. The continuous- wave electron paramagnetic resonance, CW-EPR, spectra show that five-coordinate oxo-Cr~V bischelates are formed at pH ≤ 4 with the aldaric acid bound to oxo-Cr~V through the carboxylate and the α-OH group. A different oxo-Cr~V species with Glucar was detected at pH 6.0. The high giso value for the last species suggests a mixed coordination species, a five-coordinated oxo-Cr~V bischelate with one molecule of Glucar acting as a bi-dentate ligand, using the 2-hydroxycarboxylate group, and a second molecule of Glucar with any vic-diolate sites. At pH 7.5 only a very weak EPR signal was observed, which may point to instability of these complexes. This behaviour contrasts with oxo-Cr~V-uronic species, and must thus be related to the Glucar acyclic structure. In vitro, our studies on the chemistry of oxo-Cr~V-Glucar complexes can provide information on the nature of the species that are likely to be stabilized in vivo.
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Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the li...
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Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu~(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm~(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated.
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The 1,8-bis(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-tert- butylcarbazole anion (Czx) forms monomeric, six-coordinate halide complexes of Yb(II), (Czx)Yb(X)(THF)_2 (X = I (2), Cl (3)), by metathesis of YbX_2 with NaCzx (1) or Na/Hg...
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The 1,8-bis(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-tert- butylcarbazole anion (Czx) forms monomeric, six-coordinate halide complexes of Yb(II), (Czx)Yb(X)(THF)_2 (X = I (2), Cl (3)), by metathesis of YbX_2 with NaCzx (1) or Na/Hg reduction of (Czx)Yb(Cl) _2(THF). The crystal structure of 1 reveals a polymeric chain structure in which the oxazoline ring bridges to the Na~+ of an adjacent unit. The iodo complex 2 serves as a precursor to divalent silylamide, alkyl, and phosphide complexes, (Czx)Yb(X)(THF)_n (4, X = N(SiMe _3)_2, n = 1; 5, X = CH(SiMe_3)_2, n = 1; 7a, X = 2,4,6-Me_3C_6H_2PH, n = 2; 7b, X = 2,4,6-Pr~i _3C_6H_2PH, n = 2). The X-ray structure of 4 reveals a distorted-trigonal-bipyramidal geometry with the Czx ligand occupying two axial sites and one equatorial site in a pseudo-mer coordination mode. In contrast to the typical metathesis chemistry observed with LiCH(SiMe3)2, an unusual oxidation occurs when 2 or 3 is treated with LiCH2SiMe3 to generate the previously isolated trivalent alkyl (Czx)Yb(CH_2SiMe_3)_2. Trivalent Yb complexes with the Czx ligand also display unusual redox chemistry: rapid reduction to the Yb(II) phosphides 7a,b is observed on treatment of mer,cis-(Czx)Yb(Cl)2(THF) with ArPH~- Na~+ (6a,b) or, equivalently, on treatment of (Czx)Yb(CH_2SiMe_3) _2 with ArPH_2. In both cases, oxidative coupling of the phosphide or phosphine was observed to form meso- and rac-biphosphines, ArPH-PHAr (Ar = 2,4,6-Me_3C_6H_2 (9a), 2,4,6-Pr~i _3C_6H_2 (9b)).
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Noninnocent ligands that are electronically coupled to active catalytic sites can influence the redox behavior of the catalysts. A series of (μ-dithiolato)Fe_2(CO)_6 complexes, in which the sulfur atoms of the μ-dithiolato ligan...
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Noninnocent ligands that are electronically coupled to active catalytic sites can influence the redox behavior of the catalysts. A series of (μ-dithiolato)Fe_2(CO)_6 complexes, in which the sulfur atoms of the μ-dithiolato ligand are bridged by 5-substituted (Me, OMe, Cl, t-Bu)-1,4-benzoquinones, 1,4-naphthoquinone, or 1,4-anthraquinone, have been synthesized and characterized. In addition, the bis-phosphine complex derived from the 1,4-naphthoquinone-bridged complex has also been prepared and characterized. Cyclic voltammetry of these complexes shows two reversible one-electron reductions at potentials 0.2 to 0.5 V less negative than their corresponding parent quinones. In the presence of acetic acid two-electron reductions of the complexes result in conversion of the quinones to hydroquinone moieties. EPR spectroscopic and computational studies of the one-electron-reduced complexes show electron delocalization from the semiquinones to the 2Fe2S moieties, thereby revealing the " noninnocent" behavior of these ligands with these catalysts.
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The present report deals with the synthetic approach to homo- and heteronuclear polymetallic aggregates using the Ni~II-meso-octaethylporphyrinogen complex [{#eta#~3-(Et_8N_4Ni)}{Li(thf)_2}_2] (1) as a #pi# ligand in the reaction ...
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The present report deals with the synthetic approach to homo- and heteronuclear polymetallic aggregates using the Ni~II-meso-octaethylporphyrinogen complex [{#eta#~3-(Et_8N_4Ni)}{Li(thf)_2}_2] (1) as a #pi# ligand in the reaction with metal halides. The metallation reaction occurs via the probable intermediate of an isomeric form of 1, namely 2, which should be considered an organolithium derivative. Among the polymetallic aggregates, complexes [{#eta#~5-(Et_8N_4Ni)}_2Co].2[Li(thf)_4] (6) and [{#eta#~2-#eta#~4-(Et_8N_4Ni)}_2Ni_2(#mu#-Cl][Li(thf)] (8) display a thermal and photochemical lability. Electron transfer reactions can be photochemically or thermally induced resulting in the formation of metal(0) and oxidized forms of the meso-ctaethylporphyrinogen skeleton.
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Silicon hydride, as a new efficient catalyst for CO2 conversion to CO, is receiving ever-increasing attention. Thus, a precise catalytic mechanism is needed to provide a guideline for the design of new efficient catalysts based on...
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Silicon hydride, as a new efficient catalyst for CO2 conversion to CO, is receiving ever-increasing attention. Thus, a precise catalytic mechanism is needed to provide a guideline for the design of new efficient catalysts based on silicon hydride. Qian et al. recently proposed a one-step mechanism for the catalytic process. The present work, based on the recently proposed WMS and revM06 quantum mechanical model chemistries, provides a multistep mechanism that has a lower free energy of activation than the one-step model. A key point in favor of the new mechanism is that it is consistent with experimental details of the infrared spectrum that cannot be explained by the previous mechanism.
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When excess uronic acid over Cr~(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr' yieldsD-glucaric acid (Glucar) and Cr"' as final products. The redox reaction involves the formation ofintermediate Cr~(VI) and Cr~V ...
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When excess uronic acid over Cr~(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr' yieldsD-glucaric acid (Glucar) and Cr"' as final products. The redox reaction involves the formation ofintermediate Cr~(VI) and Cr~V species, with Cr~(VI) and Cr~Vreacting with Glucur at comparable rates. The rateof disappearance of Cr~(VI), and Cr~V increases with[H~+]and [substrate]. The experimental resultsindicated that Cr~(VI) is a very reactive intermediate since its disappearance rate is much faster thanCr /Cry and decreases when [H~+]rises. Even at high[H*]Cr~(VI) intermediate was involved in fast stepsand does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucurand Cr"' proceeds through a mechanism combining one- and two-electron pathways for the reductionof intermediate Cr(iv) by the organic substrate: Cr~(VI) →Cr~(IV)→Cr~(II) and Cr~(VI) → Cr~(IV) Cr~(III). Themechanism is supported by the observation of free radicals, CrO_2~(2+)(superoxoCrion) and Cr~V asreaction intermediates. The EPR spectra show that five-co-ordinate oxo-Cr~Vbischelates are formed atpH < 4 with the uronic acid bound to Cr~V through the carboxylate and the a-OH group of the furanoseform. Five-co-ordinated oxo-Cr~Vmonochelates are observed as minor species in addition to the majorfive-co-ordinated oxo-Cry bischelates. At pH 7.5 the EPR spectra show the formation of a Crcomplexwhere the cis-diol groups of Glucur participate in the bonding to Cr. In vitro, our studies on the chemistry of Cr~V complexes can provide information on the nature of thespecies that are likely to be stabilized in vivo. In particular, the EPR pattern of Glucur-Cr~V species canbe used as a finger print to identify Cr~Vcomplexes formed in biological systems.
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Guanidino-functionalized aromatic compounds (GFAs) are strong organic electron donors and redox-active ligands. The diversity of GFA chemistry results from reaction sequences that are triggered by their oxidation. This work report...
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Guanidino-functionalized aromatic compounds (GFAs) are strong organic electron donors and redox-active ligands. The diversity of GFA chemistry results from reaction sequences that are triggered by their oxidation. This work reports on reactions of a new GFA compound, namely, 1,2,4,5-tetrakis(N,N-dicyclohexylguanidino)benzene, with transition metals, and analyzes the interplay between GFA oxidation, coordination, and hydrogen-bond formation up to deprotonation. The reaction of the new GFA with [{PdCl(C3H5)}(2)] led to coordination and a coupled proton- and electron-transfer reaction.
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The synthesis and use of the new tris-alkynyl dendrons 2 to 5 are reported,including the Williamson reaction of 5 with 9-iodo (9), 27-iodo (11),and 81-iodo (12) dendritic cores to yield 27-alkynyl (13), 81-alkynyl (14), and 243-al...
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The synthesis and use of the new tris-alkynyl dendrons 2 to 5 are reported,including the Williamson reaction of 5 with 9-iodo (9), 27-iodo (11),and 81-iodo (12) dendritic cores to yield 27-alkynyl (13), 81-alkynyl (14), and 243-alkynyl (15) dendrimers. Socalled click reactions of these three dendrimers with azidomethylferrocene (20) give 27-ferrocenyl (16), 81-ferrocenyl (17), and 243-ferrocenyl (18) dendrimers. Electrochemical recognition of oxo-anions (H2PO4 and ATP2) and Pd2+ cation has been compared using the three polyferrocenyl dendrimers. Derivatization of Pt electrodes with the dendrimers for recognition becomes more facile with increasing size of the dendrimer. This first click dendrimer bearing 243-ferrocenyl groups is the best one in the series to obtain robust, recyclable modified Pt electrodes, whereas previous click ferrocenyl dendrimers have not been suitable for this purpose.
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Aim of this contribution is to review some recent quantum mechanical approaches used to compute the redox potentials of transition metal complexes, with the emphasis on copper and iron species, which are particularly relevant in i...
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Aim of this contribution is to review some recent quantum mechanical approaches used to compute the redox potentials of transition metal complexes, with the emphasis on copper and iron species, which are particularly relevant in inorganic biochemistry and in synthetic chemistry of bio-mimetic compounds. The paper presents also new DFT results obtained on Cu and Fe aquo ions in the framework of the Thermodynamic Integration and Grand Canonical Ensemble approaches. Such results show that without explicit inclusion of water molecules in the external solvation shells (even using a continuum solvation model) also very advanced methodologies fail to predict the redox potential in an acceptable manner. This is a confirmation of some previous studies which however never addressed this specific problem along the aforementioned approaches. Better results are obtained, on the contrary, on a series of Cu(II) complexes with Gly, Ala, en, Im, and water ligands with coordination type N2O2 or N-4. In this case, the complexes are surrounded by nine water molecules which may partially alleviate the inadequacy of the continuum solvent models, especially in the case of highly positively charged species.
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